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Are we not metazoans?

Evolutionary developmental biology, or "evo-devo" for short, has become an active area in biology research over the last few decades.

One of its first eminent advocates was Ernst Haeckel (1834-1919), well-known for his principle that "ontogeny recapitulates phylogeny", or in simpler words, "growth reruns evolution". That has turned out to be a rather grotesque oversimplification, and his most famous diagram has outright errors - it makes early vertebrate embryos look too similar.

Before him, Karl Ernst von Baer (1792-1876) had noted these principles of development, though he did not accept evolution:

  1. General characters appear earlier in development than specialized characters.
  2. Less general character appear later (and build on) the general framework of earlier stages.
  3. The embryo of any organism, rather than passing through the stages of other forms, tends to progressively differentiate itself from them.
  4. The embryo of one animal form never resembles the adult of another, but only its embryo.

(An historical meme)

In the early to mid 20th cy., biologists discovered several "homeotic mutations". They make one body part develop like another:

  • Antennapedia - makes insect antennae become walking legs
  • Proboscipedia - like antennapedia, but for mouthparts
  • Bithorax - flies' halteres become wings

In the 1980's, biologists found some genes that make Drosophila melanogaster fruit flies grow segments and that give those segments their identities. The segment identity genes are better-known as homeobox or Hox genes. Some of them are named after the homeotic effects of some associated mutations, like antennapedia. Hox-associated mutations can have homeotic effects because they make Hox genes develop in the wrong places, or else change the effects of the Hox proteins. Since then, genes homologous to these genes have been found across the animal kingdom, specifying nose-to-tail identity in the same order in most species.

Plants have a somewhat similar mechanism for specifying flower parts, though with MADS-box genes instead of homeobox genes expressed along their stem-length axes.

There are also numerous homologies among genes specifying dorsoventral or back-to-belly identity, reviving an old hypothesis that vertebrates are flipped over relative to most of the rest of the animal kingdom.

  • Invertebrates: ventral, vertebrates: dorsal
  • Central nervous system
  • Gut
  • Heart
  • Invertebrates: dorsal, vertebrates: ventral

The homologies extend to various other sorts of genes, like genes involved in making eyes. This has given rise to the concept of "deep homology", that many important features are homologous across much of the animal kingdom, even if they often look very different.

External links[]

PZ Myers has blogged numerous times on evo-devo and related discoveries.



Main body axis: rostral-caudal / cranial-caudal / nose-to-tail

Other body axes: dorsoventral / back-to-belly and left-right

Eyes and other sense organs

Skeletal features, limbs, and internal organs

Genes and genomes


Conferences and broader issues